1. Field of the Invention
The present invention relates to a pattern forming method, and a method for manufacturing a semiconductor device of a liquid crystal display device or the like, and is particularly preferable for application in forming a gate electrode of an extremely fine size.
2. Description of the Related Art
With high integration density of semiconductor elements, microfabrication of patterns formed by lithography has been developed. As a method for forming a micropattern with high accuracy, a multiple exposure process using an alternating phase shift mask is proposed.
The method of the multiple exposure process will be described by using FIG. 26.
For example, when a gate layer in a logic circuit of a semiconductor device is formed, an element isolation region 101, and an active region 102 defined by the element isolation region 101 exist on a silicon substrate as shown in FIG. 26A. The gate layer is usually formed laterally across the active region 102.
On forming the gate layer, first mask patterns 104 are exposed on a photoresist (not shown) of the silicon substrate by using a first photomask 103 which is an ordinary chrome mask, an attenuated phase shift mask, or the like. Thereafter, as shown in FIG. 26B, second mask patterns 106 are exposed (double-exposure) on the photoresist by using a second photomask 105 which is an alternating phase shift mask so as to be superimposed on the first mask patterns 104.
The alternating phase shift mask is constructed so that the phases of the adjacent mask patterns are shifted by π (180°) from each other. With exposure by using the alternating phase shift mask, the light intensity becomes very steep, and a very large depth of focus can be obtained by using a relatively small illumination system. As a result, as shown in FIG. 26C, gate layers 111 which are made narrow only above the active region 102 are formed. When the gate layer is formed with double (multiple) exposure like this, an extremely large exposure margin can be obtained as compared with single exposure.
The alternating phase shift mask is capable of desired microfabrication with an extremely large margin as described above, but on the other hand, it has the great problems that its manufacture process is complicated and manufacture cost is high. The problem of a three-dimensional structure of the alternating phase shift mask cannot be neglected. On manufacturing an alternating phase shift mask, a lower part of a light shield film between 0 and π is in an eaves-shape. This is made to have the three-dimensional structure to eliminate imbalance of intensity of light passing through each opening. However, when miniaturization of the processing dimension increases, and the light shield film becomes smaller, this is expected to be a great problem in manufacture of photomasks.
In this respect, the art of Japanese Patent Application Laid-open No. 2001-126983 is proposed as a multiple exposure process. This art is the art of dividing a mask data into two on forming a pattern of one layer, and transferring the divided patterns by using illumination systems and optical conditions which are optimized for the respective patterns and by using double pole illumination as at least one illumination system. This is a multiple exposure process without using an alternating phase shift mask, and is advantageous in the respect that the manufacture cost is low. However, this method has the problem that since one photomask is used for one pattern, a sufficient manufacture process margin cannot be obtained when exposing a number of patterns.